This invention is directed generally to encoding circuits and more particularly to a tri-state encoding circuit for an electronic musical instrument.
Various tri-level or "tri-state" sensing or encoding circuits have been proposed in the prior art for various functions. Basically, such a circuit is useful in developing a signal which may be transmitted over a single line, and yet contain sufficient information to derive three distinguishable signals. For example, in an electronic musical instrument of the keyboard variety, each key may have one of three possible positions or states: unplayed or "up," played or "down," and in "transition" or between the "up" and "down" positions. In developing a percussion-type voice from such an instrument, e.g., a piano voice, it is useful to determine the position of a key at any given time, in order to develop a suitable signal indicative of the intensity of actuation of the key. In this regard, in order to electronically simulate the sound of a conventional piano, for example, it is desirable to develop a signal corresponding to the intensity of actuation of a key, since the intensity of actuation of a piano key effects the sound and tonal quality of the note played.
Accordingly, it will be seen that a signal having three states, corresponding to the three possible positions or states of a key-actuated switch may readily be utilized to develop a signal indicative of intensity of actuation of the associated key. For example, the signal may be utilized to turn on and turn off various clocks, counters or the like. Such a system is shown, for example, in Wheelwright et al U.S. Pat. No. 4,067,253.
Advantageously, the ability to develop such a tri-state signal using but a single line from the associated key-operated switch to internal musical instrument circuits results in great savings, as the number of lines required to be physically wired into the keyboard are reduced by one-half. In this regard, it will be recognized that conventional practice would call for a separate wire from the keyboard of the instrument for each fixed contact of each key switch, thereby requiring at least two wires per switch. With the provision of suitable encoding circuitry, however, only a single wire from the movable contact of each key switch need be utilized.
While various arrangements, including the aforementioned U.S. Pat. No. 4,067,253, have been proposed for producing such a signal, the present invention provides a novel and improved tri-state encoder circuit for this purpose. Advantageously, the circuit of the present invention utilizes signal sources already available in the electronic musical instrument and further utilizes but a minimum of components to develop the suitable encoded signals. Moreover, the components utilized in the present invention are of a type which may be readily realized as a part of a large scale integrated circuit. Consequently, a tri-state encoder circuit according to this invention may be provided for each of the keys of a keyboard-type musical instrument, for example, on but a single integrated circuit chip. It will be recognized that such a tri-state encoding circuit is equally useful in conjunction with any control member or switch of such an electronic musical instrument which may assume one of three possible states. Such provision would similarly reduced the number of connecting wires which must otherwise be provided from such a member to the control circuitry of the instrument.